Surface mounting technology (SMT) is a desirable method of mounting electronic components, such as resisters, capacitors, inductors, transistors, integrated circuits, chip carriers and the like, to circuit boards and other electronics substrates, and is particularly desirable for fabricating small circuit structures. In addition, surface mounting technology lends itself well to process automation and high-density electronic manufacturing, wherein surface-mountable microelectronic devices are bonded to circuit boards and electronic substrates by solder reflowing processes.
For example, a common surface-mountable microelectronic device, referred to as a xe2x80x9cflip chip,xe2x80x9d includes an integrated circuit device with numerous connecting leads attached to pads mounted on its underside. Either a circuit board, to which the flip will be mounted, or the flip chip itself is provided with small bumps or balls of solder (hereinafter referred to as xe2x80x9csolder ballsxe2x80x9d or xe2x80x9csolder spheresxe2x80x9d) that are positioned at locations on the board or the chip that correspond to the chip pads. The flip chip is mounted to the circuit board by (a) placing it in contact with the board such that the solder spheres become sandwiched between the board and the corresponding pads on the chip underside, forming an assembly; (b) heating the assembly to a point at which the solder reflows; and (c) cooling the assembly. Upon cooling, the solder hardens, thereby mounting the flip chip to the surface of the circuit board.
Tolerances in assemblies using flip chip technology are critical, as the spacing between individual microelectronic devices as well as the spacing between the chip and the circuit board is typically very small. For example, the spacing of chips from the surface of the board may be in a range of about 0.5 to about 3.0 mil (about 12.7 to about 76.2 xcexcm), and is expected to approach micron spacing in the near future.
Electrical connections are also achieved with ball grid array (BGA) packages that are made by placing solder spheres of precisely controlled diameter and unblemished surface condition between circuit pads. Solder spheres are then heated above the liquidus temperature of the solder alloy, thereby melting the solder spheres, which wet and flow onto contact pads, creating a mechanical joint and an electrical interconnection between the circuit board and mounted component.
Solder alloys that are commonly used to manufacture solder and solder spheres for reflowing and BGA soldering applications include relatively soft base metals, such as lead, tin, silver, aluminum or copper, that can be easily damaged during manufacture of solder spheres as well as during transport and storage of solder spheres. In particular, during stream-jet fabrication of solder alloy spheres, cooling and solidification of spheres are important to the formation of smooth, unblemished and homogeneous sphere surface finishes. The cooling and solidification phases of solder sphere production are typically conducted in controlled atmospheres, such as an inert atmosphere created by nitrogen gas. However, the ability of nitrogen gas to cool solder spheres rapidly enough to form the desired surface finishes is limited by the heat transfer coefficient of nitrogen and the absolute difference between the temperatures of nitrogen gas and molten solder droplets. This limited heat transfer causes metallurgical phases to segregate on surfaces of cooling solder droplets, forming blemished surface finishes on the solidified solder spheres. In addition, the relatively slow rate of cooling facilitates the growth of large or coarse metal grains that impair surface finishes of solidified spheres.
Damage to surfaces of solder spheres can have a number of consequences. For example, automated vision systems may not be able to distinguish a solder sphere from the background if the surface reflectivity of the solder sphere has been diminished due to surface damage. Physical surface damage will also hinder the ability of most automated BGA assembly hardware to pick and place individual solder spheres. In addition, the presence of extraneous particles or grains on solder sphere surfaces may impair the mechanical function of the BGA assembly hardware. Extraneous particles and grains may also cause low resistivity or electrical shorts between contact pads on the microelectronic devices and circuit boards or other electronic substrates, as well as compromise the electrical performance of BGA joints once formed. Finally, surface damage exacerbates the oxidation of base metals of solder sphere surfaces. Surface oxidation impairs proper wetting and flow of solder spheres onto contact pads, as required with BGA packages to form reliable mechanical joints and electrical interconnections.
Existing methods of manufacturing metal spheres and fine metal powders include atomizing and centrifuging techniques, wherein a metal melt is finely divided to form molten metal droplets. Molten metal droplets are often contacted downstream of an atomizing or centrifuging location with a coolant medium to cool and solidify droplet metal into particles and spheres. Typically, the coolant medium is a gas. In some cases, others have introduced a mist in the vicinity of the atomizer to extract heat from the coolant gas.
In accordance with methods that are more-fully described herein, solder balls are formed by flash vaporizing a liquid coolant from the surface of atomized molten solder as the atomized molten solder is sprayed into a mist of the liquid coolant in an inert gas to cool and solidify the solder balls. The molten solder can be atomized with a stream jet, and the cooling liquid can be introduced via a mist-generating nozzle
The liquid coolant can be made to contact the molten solder ball surface and flash vaporize thereon (rather than simple evaporation of the liquid coolant as a consequence of its exposure to the hot gas in the chamber) by, e.g., regulating the temperature and/or composition of the liquid coolant, regulating the droplet particle size of the liquid coolant and controlling the density of liquid coolant droplets in the mist or aerosol. Generally, the optimal and/or most-efficacious surface contact of the molten solder balls with the liquid coolant can be achieved by any combination of decreasing the temperature of the liquid coolant, producing smaller droplets of the liquid coolant in the mist and increasing the droplet density of the mist immediately adjacent to the molten solder ball surfaces. Ideal values for each of these variables are interdependent and also dependent on the temperature, size and composition of the atomized molten balls; and the adjustments, described above, should not be carried out to an extent that would prevent the liquid coolant from xe2x80x9cflashxe2x80x9d vaporizing upon contact with the molten solder ball.
The methods described herein provide a rapid and efficient method of cooling the surface of a solder melt. The flash vaporization of the liquid coolant droplets and the consequent rapid cooling of the solder balls can produce solidified solder spheres having very-fine grain size at their surfaces and improved sphericity. These methods can also be used to suppress formation of metal oxides on solder ball surfaces and to suppress the segregation of phases in the solder ball. Resultant solder spheres consequently can have smooth, blemish-free and homogeneous surface finishes in addition to having a very narrow particle size distribution. The enhanced control over the character and quality of the solder spheres offered by these methods is particularly advantageous when they are used in microelectronics featuring very-small spacing of components and having very-fine dimensional tolerances. Moreover, the enhanced cooling of the solder spheres provided by the flash vaporization enables a smaller apparatus with a smaller atomization chamber to be used because the solder spheres need less time (and a shorter path) traveling through the inert gas before their surfaces are sufficiently solidified to contact a collection surface without damaging their shape.